1. Field of the Invention
The present invention relates to a composition containing a phosphonitrile halide catalyst and also linear polydimethylsiloxanes having a defined MW range and defined chain length for depolymerization and removal of silicone residues. The composition of the present invention is particularly useful for removing crosslinked or uncrosslinked, filled or unfilled silicone residues from sensitive surfaces and substrates. The silicone dissolver can be liquid, pasty or gel-like in the various applications, depending on the field of use.
2. Description of the Related Art
Various methods and compositions for removing or depolymerizing crosslinked or uncrosslinked silicones are known in the prior art, for example                Swelling and removing with an organic or halogenated organic solvent;        Swelling and removing with an amine such as diisopropylamine;        Depolymerizing the polydimethylsiloxane with alkaline solutions of strong bases such as sodium hydroxide or potassium hydroxide as described for example in U.S. Pat. No. 2,710,843;        Depolymerizing with short-chain or cyclic oligodiorganosiloxanes as described for example in DE 69122740 T2;        Depolymerizing with aqueous solutions of strong acids such as for example alkylbenzenesulphonic acids, sulphuric acid, hydrofluoric acid as described for example in JP 8250400 A; and        Depolymerizing with a composition containing phosphonitrile halide compounds and short-chain oligodimethylsiloxanes of up to 7 Si-chain members as described for example in EP 0 884 368 A1.        
The contrary use of cyclic or linear phosphonitrile halide compounds as chain growth addition or condensation polymerization catalyst to prepare long-chain polydimethylsiloxanes is long-established prior art, as is using the catalyst to equilibrate polydimethylsiloxanes as described in DE 196 07 264 A1.
U.S. Pat. No. 5,008,229 is directed to solutions of phosphonitrile chlorides in an organic solvent and the use of such compositions to accelerate the condensation and/or equilibration of hydroxyl-terminated diorganosiloxanes. Although improved results are obtained, an organic solvent such as ethyl acetate is used to facilitate the dissolution of the phosphonitrile chloride. DE 37 25 377 eliminates the use of an organic solvent by using a reaction product of a phosphorus-nitrogen chloride and of a cyclic diorganosiloxane, such as octamethylcyclotetrasiloxane, as catalyst. Experience has shown, however, that reaction products of phosphorus-nitrogen chloride and octamethylcyclotetrasiloxane frequently have viscosities exceeding several thousand mPa·s (centipoises), making the materials less useful as catalysts for removing polydimethylsiloxane residues.
The disadvantage of all silicone dissolvers heretofore available is that they contain solvent or contain corrosive substances such as acids or alkali which, when used to remove silicones from diverse plastics substrates for example, attack these substrates or cause the substrates to swell. Other surfaces such as metals or semiconductors for example may suffer corrosive attack.
The problem addressed by the present invention was therefore that of providing a composition that is useful for removing crosslinked and uncrosslinked polysiloxane residues from various substrates or from between substrates, but is inert with regard to the substrates and does not damage, corrode or swell them even as a thin layer. There are many applications where it is especially the swelling of thin substrates which plays a decisive part when the silicone-dissolving composition is used in processes where it comes into direct contact with other plastics. A surprising finding was a direct relationship between the chain length of the linear siloxanes used and the swelling of plastics present as a thin layer, particularly below a layer thickness of 250 μm.